The present invention pertains generally to control systems and more particularly to antenna positioning in tracking radars. An important aspect of controlling antenna movement in tracking radars is correcting the traverse error signal as the elevation angle varies to obtain the proper azimuth position of the tracking radar. The problem is produced by the fact that the traverse error signal produced by the radar tracking electronics is proportional to the traverse plane trajectory of the target being tracked. Although the traverse plane trajectories of two separate targets, one at the horizon and the other at a very high elevation angle, such as 70.degree., require the same change in azimuth movement of the radar antenna, the traverse plane trajectory of the elevated target is much smaller than the traverse plane trajectory of the target on the horizon. Since the traverse error signal produced by the radar is proportional to the traverse plane trajectory detected by the radar, the azimuth error signal must be amplified as the elevation angle is increased to obtain the true azimuth error signal.
The ratio of angular displacement at high elevation to the displacement at the horizon (zero elevation) is equal to the cosine of the elevation angle. The true azimuth angular error is therefore equal to the traverse error detected by the receiver divided by the cosine of the elevation angle. Since the secant function is equal to 1/cosine, the true azimuth error signal is equal to the traverse error signal produced by the radar times the secant of the elevation angle. Therefore, the correction factor required to obtain the true azimuth angular error signal is proportional to the secant of the elevation angle, and this function must be approximated in some manner to produce the proper correction factor.
The conventional manner of generating the correction factor has been to mechanically connect a secant wound potentiometer to the vertical gear train of the radar antenna pedestal. The traverse error signal is then applied to the terminals of the potentiometer to produce the true azimuth angular error signal, since the output taken from the wiper of the pot varies in amplitude proportionally to the secant of the elevation angle.
While this type of system is very simple and straightforward in concept, it suffers from various disadvantages and limitations. In operation, the secant pot must be physically located on the pedestal so that it is mechanically coupled to the antenna gear train. Since the traverse error signal is generated from the radar electronics which are physically separated from the antenna pedestal, the traverse error signal must be run over cable for long distances which tends to introduce noise resulting in poor tracking signals. Additionally, the error signal must be routed through slip rings in the pedestal which also add noise to the azimuth error signal.
Another problem with this type of system is that secant wound potentiometers are expensive and are becoming increasingly difficult to obtain. This results in extended "down-time" for the radar system if the potentiometers cease functioning.
As with most precision potentiometers, the functional accuracy of the secant potentiometer is only maintained when loaded with high impedance since, as the load resistance decreases, the functional error of the output increases. A cable between the radar electronics and the pedestal carrying the traverse error signal must therefore be terminated with a high impedance to maintain proper accuracy. The combination of a long line and high impedance, of course, renders the system more susceptible to noise pickup, even with the use of shielded cables. The signal-to-noise ratio is even further degraded by low error signal amplitudes typically encountered in the automatic track mode.
Finally, the design of the potentiometer itself introduces other disadvantages and limitations. For example, the movement of the wiper across the wire windings produces both electrical noise and mechanical wear. The performance of the potentiometer is slowly degraded by this wear as well as environmental factors due to the location of the potentiometer on the pedestal. The performance of the system eventually reaches unacceptable levels requiring placement of the secant potentiometer at a substantial cost.